You can render bigger than you need to and shrink fit down to increase resolution if you need. The most important thing is your lighting & render settings, imho. But yeah exact resolution is the size you want. I gives you more freedom and various sizes to have larger scenes like landscapes. As you set up your scene in an exact size render, the areas not seen in the output will be dulled out. I prefer exact resolution renders because I can control the image size.

Have a creative day!

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